Impairment of Biofilm Formation by TiO2 Photocatalysis through Quorum Quenching

The release of nanomaterials into the environment, due to their massive production and application today, has caused ecological and health safety concerns. Semiconductor photocatalysts like TiO₂ exhibit cytotoxicity to bacterial cells when exposed to UV irradiation. However, information about their impacts on individual or group bacterial behaviors is limited. In this work, the biofilm formation of Escherichia coli K12 in the presence of TiO₂ with and without UV irradiation was investigated and biofilm formation was found not to be affected under the sole application of TiO₂ or UV irradiation. However, biofilm development was substantially delayed by TiO₂ under UV irradiation, although no obvious cytotoxicity to cell growth was observed. The reactive oxygen species photogenerated by TiO₂ were found to quench the autoinducer 2 (AI-2) signals secreted by E. coli K12. As a result, the initiation of quorum sensing for biofilm formation activated by AI-2 was restrained. The expressions of two biofilm-formation-related genes, motA and rcsB, were also suppressed. A dose of an AI-2 precursor, 4,5-dihydroxy-2,3-pentanedione, effectively restored the biofilm development. These results show that the photoexcited TiO₂ could suppress biofilm formation through quenching AI-2 signals. This work may facilitate a better understanding about the ecological effects of increasingly released nanomaterials and provide implications for development of antifouling membranes.

Polydopamine-graphene oxide derived mesoporous carbon nanosheets for enhanced oxygen reduction

Composite materials combining nitrogen-doped carbon (NC) with active species represent a paramount breakthrough as alternative catalysts to Pt for the oxygen reduction reaction (ORR) due to their competitive activity, low cost and excellent stability. In this paper, a simple strategy is presented to construct graphene oxide-polydopamine (GD) based carbon nanosheets. This approach does not need to modify graphene and use any catalyst for polymerization under ambient conditions, and the obtained carbon nanosheets possess adjustable thicknesses and uniform mesoporous structures without using any template. The thickness of GD hybrids and the carbonization temperature are found to play crucial roles in adjusting the microstructure of the resulting carbon nanosheets and, accordingly their ORR catalytic activity. The optimized carbon nanosheet generated by a GD hybrid of 5 nm thickness after 900 °C carbonization exhibits superior ORR activity with an onset potential of -0.07 V and a kinetic current density of 13.7 mA cm(-2) at -0.6 V. The unique mesoporous structure, high surface areas, abundant defects and favorable nitrogen species are believed to significantly benefit the ORR catalytic process. Furthermore, it also shows remarkable durability and excellent methanol tolerance outperforming those of commercial Pt/C. In view of the physicochemical versatility and structural tunability of polydopamine (PDA) materials, our work would shed new light on the understanding and further development of PDA-based carbon materials for highly efficient electrocatalysts.